A correlation value of two images may be used for detecting movement, rising, falling, inclination, changes in form and immovability of a surface. If the images are similar, the surface has remained unchanged and the correlation value is high. If, on the other hand, there are differences between the images, the surface has changed in some way, which decreases the correlation value. Searching for the maximum value of correlation by moving the images to be compared with respect to each other in time or in terms of location may provide information on the movement of the surface to be measured.
The movement velocity of a surface may also be measured by creating image rows of the moving surface in the direction of movement and by arranging the image rows into an image matrix. The angular coefficient of the lines in the matrix may be used for determining the surface velocity. Such a solution is described in Finnish Patent 80527.
However, the prior art solutions involve problems. Correlation requires that comparable images exist on the surface to be measured. In addition to the use of correlation and various velocity measurements, it is difficult or impossible to measure a (nearly) immovable surface, a surface that moves back and forth, or a surface that moves irregularly because the surface movement does not always have a velocity that can be determined unambiguously.